1. Field of the Invention
The present invention relates to an information recording apparatus and method which use a physical phenomenon caused by applying a voltage between a probe and a sample while they are kept close to each other.
2. Description of the Related Art
Recently, a scanning tunneling microscope (to be referred to as an STM hereinafter) has been developed (G. Binning et al., Helvetica Physica Acta, 55, 726 (1982)), which allows direct observation of electron structures on and near the surface of a substance by using a physical phenomenon (tunnel effect or the like) appearing when a probe is brought close to a sample. With this microscope, actual spatial images of single crystals and amorphous substances can be measured with high resolutions.
In addition, the STM allows observation with low power without causing damage due to currents to a medium. Furthermore, the STM can operate in the atmosphere and in a solution as well as in an ultra-high vacuum, and can be used for various types of materials. A variety of applications of the STM are therefore expected in various academic and research fields.
In the industrial field as well, attention has recently been given to the principle of the STM having a spatial resolution at an atomic or molecular level efforts have been made to apply the STM to information recording/reproducing apparatuses using a medium having a recording medium and to put it into practical use, as disclosed in Japanese Laid-Open Patent Applications Nos. 63-161552 and 63-161553.
In these conventional methods, an SOAZ.multidot.Langmuir-Blodgett (LB) film stacked on an Au electrode is used as a recording medium.
In a recording operation, a voltage on which a pulse wave is superimposed by using a bias applying circuit between the probe and the medium is applied between the sample and the probe.
In a reproducing operation, while a predetermined DC voltage is applied between the probe and the medium, the medium surface is scanned with the probe to detect current changes.
For example, by this method, a low-resistance portion A is formed in a high-resistance LB film on the medium, as schematically shown in FIG. 1.
Assume that in a reproducing operation, a predetermined DC voltage is applied to the Au electrode, and a current flowing through the probe is measured. In this case, almost no current flows while the probe is scanning/moving over a high-resistance portion, but the current value abruptly increases when the probe moves across the recorded bit portion. By detecting this change in current value, the presence/absence of a bit can be detected.
As described above, data is recorded on this medium by applying a voltage pulse thereto using the probe. The peak value of the voltage applied for a recording operation must be equal to or larger than a threshold.
This threshold voltage, however, varies owing to local variations in the thickness of an organic film used as a medium, thus posing a problem.
Transition is caused by a high bias at a thick film portion, but is caused by a low bias at a thin film portion. If a pulse peak value is determined with this threshold being kept constant, no transition occurs at a thick film portion, but breakdown occurs in a thin film portion owing to an overcurrent. As a result, the probability of forming stable bits decreases.
If, for example, a sufficiently high voltage is determined as a peak value in consideration of such variations in film thickness, an excessive current is applied to a bit on a portion with a small threshold. As a result, the formed bits become unstable in terms of bit shape, size, position, and the like.
Consider the maximum current in this case. When the current is equal to or smaller than a given current value, a stable bit having a size of about 10 nm, reflecting the shape of the tip of the probe, can be obtained. When the current is larger than this current value, breakdown is caused in the medium by the current. As a result, a structure having a size of about several 100 nm is formed, with the bit shape being irrelevant to the shape of the tip of the probe. The resistance of the organic film of a portion on which a bit is recorded is smaller by 100 times or more than that of a portion on which no bit is formed. For this reason, if a given constant voltage is applied to the medium, and the voltage is not a proper value, the resistance of the voltage-applied portion decreases, and at the same time, a current flowing through a resistor R.sub.T in the medium portion abruptly increases to exceed a current threshold I.sub.E at which breakdown occurs, and reaches a saturation current value Isat, as shown in FIG. 2. As a result, a bit is formed with breakdown, exhibiting instability in terms of shape and position.
Furthermore, the error correction function, the tracking function for a read operation, and the like must be enhanced because of these problems in a recording operation, resulting in a heavy load on the system. In addition, the error rate in a recording/reproducing operation is high.